The background description provided here is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
When a motor vehicle is towing a trailer, a situation may arise in which the trailer oscillates or sways back and forth. This situation, commonly referred to as trailer sway, may become uncontrollable. Unless stabilized, trailer sway may be hazardous to vehicles in adjacent lanes, or in more severe circumstances may push the rear end of the leading vehicle (i.e., the vehicle performing the towing) laterally and cause the leading vehicle to fishtail and stray from the intended path.
Trailer sway frequently occurs when a leading vehicle and accompanying trailer are travelling too fast for a given set of road and/or environmental conditions. The difference of a few miles per hour in vehicle speed can have a significant impact on the sway of a trailer. When trailer sway occurs, it can be difficult for the driver of the leading vehicle to respond correctly to stabilize the situation. This may be due to inexperience and/or panic of the driver. The driver may forego applying the vehicle's brakes or, worse yet, may abruptly apply the brakes, such as what might be done during an emergency braking action. Such abrupt application of the brakes, at the wrong time, may cause the trailer to sway even more and/or possibly influence a direction of travel of the rear wheels of the leading vehicle, ultimately leading to jackknifing.
FIGS. 1 through 8 illustrate the angular velocity and angular acceleration of a trailer 12 swaying about its tongue, which is connected to a towing vehicle 10. The figures show the full range of the trailer's 12 swaying motion of travel. FIG. 1 assumes that the trailer 12 has swayed from a position far to the left of the vehicle's centerline 14 (i.e., from the driver side of the vehicle 10) and has reached the vehicle's centerline. Angular acceleration of the trailer is zero at this point, angular velocity is at a maximum value.
FIGS. 2-5 illustrate how the yaw acceleration of the vehicle 10 continuously increases until a maximum angular point of travel of the trailer 12 is reached (FIG. 6). Over the same arc of travel, the angular velocity of the trailer is continuously dropping, with the angular velocity being essentially zero at the point of angular travel shown in FIG. 6. From the point shown in FIG. 6, as the trailer 12 sways back toward the centerline 14, yaw acceleration of the vehicle 10 drops from a maximum value while the angular velocity increases, essentially as shown sequentially in FIGS. 6, 5, 4, 3, 2 and then 1, but with the Acceleration and Velocity arrows reversed in their directions. The same pattern repeats itself when the trailer 12 sways past the vehicle centerline 14 on the driver's side of the vehicle 10. FIG. 8 illustrates that the maximum yaw acceleration of the vehicle 10 is reached at a maximum point of angular travel of the trailer 12 on the driver's side, with the angular velocity of the trailer being essentially zero at this point. From the point shown in FIG. 8, the trailer 12 will begin swaying back to the vehicle centerline 14 as shown in FIG. 9. Thus, when swaying back to the vehicle's centerline 14, the yaw acceleration of the vehicle 10 will decrease while the angular velocity of the trailer 12 increases. The yaw acceleration of the vehicle 10 again reaches zero, and the angular velocity of the trailer 12 reaches a maximum, when the trailer 12 reaches the vehicle's centerline 14. This is shown in FIGS. 3, 2 and 1 but with the direction of change in magnitude being opposite to what is shown in these Figures.
Many current systems for stabilizing trailer sway operate to apply braking to one or two wheels on one side of the vehicle at a time, depending on the exact angular position of the trailer. Such systems attempt to eliminate trailer sway by inducing a moment on the leading vehicle to counteract the yaw moment caused by the tongue of the trailer acting on the receiver hitch of the vehicle. While such systems have proven to be useful for their intended purpose, a continuous need for improvement remains for the effectiveness. For example, a modern vehicle with an Electronic Stability Control (ESC) system already applies braking to one or two wheels on a side of the vehicle to stabilize it when excessive yaw is detected, even without specifically detecting trailer sway. This type of trailer sway control can only achieve minor modifications to the conventional ESC's yaw control and the modification is quickly limited by surface friction on slippery surfaces if the wheels are braked into slip. Some current systems have added overall vehicle braking on all four wheels to slow the vehicle and trailer down. Lower speed is known to promote trailer stability, but such systems typically activate with a very small amount of active braking. As explained above, abrupt braking can worsen trailer sway; and more four-wheel braking can limit the amount of side to side braking due to limited surface friction.